Separation of the elements of air



Oct. 29, 1963 COMPRESSOR EXPANDER W. DENNIS SEPARATION OF THE ELEMENTS OF AIR Filed Aug. 10, 1960 PRESSOR INVENTOR.

DEN N IS WOLCOTT COMPRESSOR United States Patent 3,108,367 SEPARATION OF THE ELEMENTS 0F AR Wolcott Dennis, Basking Ridge, N 5., assignor to Air Reduction Company, Incorporated, New York, N.Y., a corporation of New York Filed Aug. 10, 1968, Ser. No. 43,635 3 Claims. (Cl. 62-22) This invention relates to the separation of the elements of air by liquefaction into a plurality of high purity liquid products simultaneously produced.

The principal object of the invention is to obtain a high argon recovery in excess of 80 percent of the available argon in the air, in addition to simultaneous large-scale production of high purity liquid nitrogen and high purity liquid oxygen,

It is known that an argon concentrate may be obtained by processing an oxygen rich liquid which collects in a suitable portion of a pressure nitrogen rectification column. The rich liquid is expanded and then rectitied in a low pressure oxygen column, equipped with an oxygen reboiler connected thereto. From this rectification may be obtained a high purity liquid oxygen product together with gaseous nitrogen which is usually termed waste nitrogen, and which is commonly used as a refrigerant in the initial cooling of the air stream. This rectification is effected by means of liquid nitrogen reflux and high pressure, high purity nitrogen efliuent from a primary rectification step to boil the high purity liquid oxygen in the reboiler.

At the proper point in the oxygen column, an argon concentrate containing principally argon and oxygen is passed to an argon column or attachment for separation of crude argon.

To obtain the desired high argon recovery, the nitrogen reflux liquid introduced into the low pressure oxygen column desirably is a high purity nitrogen liquid containing less oxygen and argon than would be in equilibrium with the waste nitrogen eflluent. This can be accomplished by maintaining the oxygen content of the nitrogen reflux liquid at about 0.05 mol percent and the argon content at about 0.3 mol percent. A higher purity nitrogen reflux liquid can of course be used if desired.

The rate of flow of high purity nitrogen liquid reflux to the low pressure oxygen column is also important and it has been found that a liquid nitrogen reflux "corresponding to at least about 38 mol percent of the total air feed in the low pressure oxygen column is efleotive to obtain the high argon recovery which is desirable. An amount of liquid nitrogen reflux in excess of 38 mol percent can be used; however, nitrogen reflux of about 50- mol percent of the total air feed is usually the most that can advantageously be obtained in the liquefaction process. Increasing the amount of nitrogen reflux is known to result in an increase in the concentration of the argon at an intermediate level in the oxygen column and a corresponding increase in the percentage of argon recovery.

Removal of liquid nitrogen as a product from the process stream reduces the amount of liquid nitrogen (relatively to the total air feed) which is available for refluxing the oxygen column. Nitrogen product removal in the form of high purity liquid nitrogen also results in a nitrogen reflux liquid of reduced purity if it is attempted to maintain a high reflux rate. Equilibrium of nitrogen, oxygen and argon in the effluent at the top of the oxygen column then occurs at a larger proportion of oxygen and argon relatively to the nitrogen, thereby carrying to waste with the waste nitrogen a considerable amount of argon from the top of the column, which argon is thus not available at the intermediate level where greatest concentration of argon relative to oxygen occurs. The reduction in the amount and purity of the nitrogen reflux 3,198,867 Patented Oct. 29, 1963 Ice thus both result in a lowering of the percentage recovery of argon.

To permit the removal of substantial amounts of high purity liquid nitrogen product while at the same time retaining the benefits of high argon recovery, the novel and advantageous overall process herein described for the separation of the elements of air includes drawing oil and compressing a portion of the waste nitrogen from the top of the oxygen column, cooling the compressed nitro gen as in a Water cooler and by heat exchange with itself before compression, l-iquefying the compressed and cooled waste nitrogen by heat exchange with the liquid oxygen from the bottom of the oxygen column or other suitable heat exchange, and adding such liquefied waste nitrogen, together with whatever portion of the condensed high purity efliuent of the nitrogen column is not drawn oil as a product, as reflux at the top of the oxygen column, preferably after subcooling by heat exchange with the waste nitrogen effluent.

The process disclosed herein is adapted to produce simultaneously substantial amounts of high purity liquid nitrogen, high purity liquid oxygen, and liquid argon equal to percent or more of the argon in the processed air, as well as a neon-rich gaseous product and a liquid product rich in krypton and xenon.

Other objects, features and advantages will appear from the following more detailed description of illustrative embodiments of the invention, which will now be given in conjunction with the accompanying drawing.

Referring to the drawing, reference numeral 11 is applied to the air inlet conduit. Conduit 11 passes air to compressor 13 where the air is compressed to about p.s.i.a. Thereafter the compressed air is cooled to about 80 F by conventional coolers (not shown). The compressed air after flowing from compressor 13 through conduit 15, reversing valve 17, and conduit 19 enters passage 21 of reversing exchanger 23. In flowing through passage 21 the air is cooled to approximately its liquefaction temperature (about 2.80 F.) by heat exchange with counter-current flowing streams of waste nitrogen in passage 25 and a recycle nitrogen in passage 27 of reversing heat exchanger 23. Exchanger 23* is comprised of two reversing passages 21 and 25 and one nonreversing pasage 27 and is suitably designed to effect eflicient indirect heat exchange between the respective streams of air, waste nitrogen, and recycle nitrogen. In accordance with the illustrated setting of valve 17, the waste nitrogen is shown as leaving passage 25 of exchanger 23 by means of conduit 28, passing through valve 17, and being discharged to the atmosphere. As is conventional in the art, means are provided to reverse or alternate the flow oi air and waste nitrogen in the passages 21 and 25 in order that removal of carbon dioxide and other impurities which are deposited from the incoming air can be accomplished by the waste nitrogen in addition to the indirect cooling of the air by waste nitrogen. This waste nitrogen is derived in the separation process in a manner to be explained hereinafter.

. The means for'reversing the flows of waste nitrogen and air in passages 25 and 21 comprise the reversing valve 17, a timing system (not shown) and suitable check valves 29 in the various conduits leading from and to the cold end of the reversing exchanger 23-. The timing system can be any conventional means suitable for properly operating the reversing valve 17 On a predetermined time cycle and, in the interest of clarity, has not been shown.

Two of the previously mentioned check valves 29 are located in the two conduits 31 and 33 which are shown respectively as passing air from exchanger 23 toward the separation process and as bringing waste nitrogen from the separation process to the exchanger 23. A branch conduit having a check valve 29 joins waste nitrogen conduit 33 upstream from its check valve and extends to and joins the air conduit 31 upstream from its check valve. Another branch conduit .36 provides part of the alternate flow path for air from a point downstream of the check valve in conduit 33 to a point downstream of the check valve in conduit 3-1. With this arrangement it is apparent that air can alternately flow through passages 21 and 25 of the exchanger upon reversal of valve d7 and that waste nitrogen flows in the passage not being used by air.

After being cooled in exchanger 23, the air vapor flows to the high pressure column 37, enters the lower scrubber section 39 thereof where the air is at about 94 p.s.i.a. and passes up through conventional contact trays counter-current to oxygen enriched liquid air. This scrubbing operation removes any traces of high boiling constituents or impurities from the air, which traces may have passed through exchanger 23. The scrubber liquid in the bottom of the scrubber is boiled or revaporized by the fluid in boiler 41 and a small portion of the scrubber liquid having the impurities concentrated therein is purged from the bottom of the scrubber 39 by means of valved conduit '43. It is to be noted that by the action which occurs in scrubber 39, krypto and xenon in the air are removed therefrom and pass out of scrubber 39 in the purge liquid pasing through conduit 43, rather than flowing on through the process. This purge can be suitably treated to obtain the krypton and xenon, if desired, as a separated rare gas product.

Air vapor from the scrubber 39 passes up into the nitrogen rectifier section 45 of the high pressure column 37 where it is rectified into high purity nitrogen efiluent by means of conventional contact trays and the liquid nitrogen reflux which enters the top of the nitrogen rectifier 45 through pipe 47. Due to this rectification, liquid air enriched in oxygen (rich liquid) collects in annular trough 49 in the bottom of rectifier section 45. Part of this liquid is used for the above-mentioned scrubbing operation and enters the scrubber 39 by means of valved pipe 51 extending from a conduit connected to the top level of the trough 49 to the scrubber 39. Part way down the rectifier 45, liquid nitrogen (which is of high purity but not as pure as the nitrogen vapor in the top of the rectifier) results from the rectification and part of this nitrogen is collected in a suitable trough 54, disposed at an intermediate level of rectifier 45.

From the foregoingit is clear thatthe products of the nitrogen rectifier 45 are high purity nitrogen gas in the top of the rectifier, less pure liquid nitrogen in trough 54 and oxygen-enriched liquid air in trough 49. Except for the portion of enriched liquid air which passes through conduit 51 and is used in scrubber 39, this rich fluid is passed through conduit 55, its expansion valve 57, the argon apparatus (including the reflux condenser of argon attachment 140) and conduit 58 to the upper portion of the oxygen column 59. This fluid is fed at about 18 p.s.i.a. and is rectified in oxygen column 59, having the conventional tray-contact construction, into waste nitrogen vapor in the top and a pool of liquid oxygen in the bottom. This liquid oxygen at about 29l F. is Withdrawn with the raid of gravity from the bottom of rectifier 59 by means of conduit 61 and is passed to the oxygen reboiler 63 Where it is partially reboiled to give high purity liquid oxygen. The vapor which is thus formed is returned to oxygen column 5& by means of conduit 65 which empties into the oxygen column adjacent the bottom but above the pool of liquid oxygen therein. The net liquid oxygen produced in reboiler 53- is removed therefrom by conduit 67 and a suitable liquid oxygen pump 69 as the high purity liquid oxygen product of the process.

Reflux for the rectification which occurs in oxygen column 59 is a mixture of streams of liquid nitrogen from three sources. One source is the line 47 from which liquid nitrogen flows through line 53 to passage 7 4- of subcooler d 75 where it is subcooled by waste nitrogen in passage 76 from the oxygen column prior to its refluxing. A second source is partially purified liquid nitrogen obtained from the trough 54. A third source is recycled Waste nitrogen which is liquefied and supplied to the reflux subcooling of the liquid nitrogen which refluxes the oxygen column and Where it also subcools the liquid nitrogen in passage 78 of subcooler 75 which can be withdrawn as a high purity of liquid nitrogen product by means of valved pipe 97. Thereafter this waste nitrogen at about -290 F. flows by pipe 3-3 to the reversing exchanger 23 to effect the previously described refrigeration of incoming air.

The flow of waste nitrogen in conduit 33 may be supplemented when desired by introducing flash nitrogen from storage tanks by way of conduit 210 which joins conduit 33 immediately upstream from the check valves 29. By this means refrigeration may be recovered that would otherwise be lost.

Considering now the production of product liquid nitrogen from the high purity nitrogen at about ,285 F. which is produced in the top of the nitrogen column 37, it can be seen on the drawing that a conduit 83 extends from the top of high pressure column 37 to the interior passage 85 of the oxygen reboiler 63 so that the high purity nitrogen is partially condensed by contributing to the boiling of the liquid oxygen in the space which surrounds the interior passages 85 and 86 of the oxygen reboiler. After this partial condensation the nitrogen is transferred by pipe 87 to the nitrogen condenser 89. Condensation of this nitrogen is completed in condenser 89 by heat being abstracted by refrigeration recycle nitrogen at about p.s.i.a. in the upper section 91 of the condenser 89. This condensed liquid nitrogen is thendelivered by conduit 93, liquid nitrogen pump 95, and conduit 47 as reflux to the top of high pressure column 37, through conduit 53 to subcooler 75 to contribute to the reflux of the oxygen column 59, and through branch conduit 56 and subcooler passage 78 to valved product pipe 97 through which the liquid nitrogen product may be drawn ofi in any desired amount without interfering with the high argon recovery, since a special nitrogen recycle is provided as described herein to supply additional high.

purity nitrogen reflux as required to compensate for liquid nitrogen product removal.

To make up for the product liquid nitrogen removed from the system through pipe 97, a portion of the waste nitrogen passing out of the subcooler 75 through passage 76 is led off through conduit 200w passage 201 of a counter-current heat exchanger 202 and thence to a com- Q pressor 204 where it is compressed to about 75 p.s.i.g. The compressed nitrogen is returned through a water cooler (not shown) and through passage 203 of exchanger 262 where it is cooled by heat exchange with the uncon1- pressed gas. From the exchanger 202 the cooled nitrogen is passed by conduit 206 to the inner passage 86 in the oxygen reboiler 63 where it condenses. It will be noted that this condensation may be accomplished at any other suitable point in the system, for example, a second coil may be provided at the bottom of the high pressure column 37 in the vicinity of the reboil coil 41 instead of i using the coil 86 in the position shown. From the condensing coil 86 or other condensingdevice the liquefied nitrogen is passed through conduit 208 whichjoins with conduit 53 near the entrance to passage 74 of subcooler 75.

v The flow of liquid nitrogen in conduit 208 may be adjusted to compensate more or less exactly for the flow of product liquid nitrogen removed through conduit 97 in order to maintain the quantity and purity of the liquid nitrogen reflux supplied to the top of the oxygen column 59. The balance, however, need not be exact. More product liquid nitrogen may be withdrawn than is equivalent to the waste nitrogen pumped through conduit 208, within limits, with small loss of argon. Less liquid nitrogen product can be withdrawn than the cycle equivalent replaced, with a small increase in argon recovery. In the latter case, the column loading would increase and if overload resulted, the amount of waste nitrogen recycled would have to be reduced.

Because the purity of the waste nitrogen gas at the top of the oxygen column 59 may be comparatively low, it is not feasible to add the nitrogen from the top of column 59 directly to the nitrogen gas from the top of the nitrogen column 37. Instead the waste nitrogen from column 59 must be separately condensed and then added to the nitrogen from column 37, as above described.

Referring now to the argon attachment to the oxygen column, it can be seen that the argon attachment 140 is arranged to withdraw an argon concentrate from the oxygen column 59 at a point about one-half of the distance up from the base of the column or at a point in the column where the argon concentration is at least about mol percent in the vapor phase. This withdrawn argon concentrate passes through passage 142 and enters the argon column 140 which also has the previously mentioned liquid-vapor contact trays in the major portion thereof. In the top of the column a condenser 144 receives expanded rich liquid produced in the high pressure column 37 and collected in the trough 49. By mens of condenser 144 and the usual bubble cap tray construction, the argon concentrate is rectified into enriched oxygen in the base of the argon column while crude or raw argon (at about 301 F.) containing a small percentage of oxygen is removed from the top of the argon column 140 by conduit 145. Conduit 145 passes the raw argon to the raw argon liquefier 147 which is also cooled by the expanded rich liquid from the high pressure column. The rich liquid for the raw argon liquefier 147 of the argon column flows from the high pressure column 37 through conduit 55 and valve 57 from the annular'trough 49. This rich liquid is warmed in passing through passage 149 of liquefier 147, countercurrently to the crude argon in passage 151 which is being liquefied; Separate conduits 154, 155 at appropriate levels for liquid and vapor phases respectively are pro vided between the top of passage 149 and the input to condenser 144 as are also provided separate conduits 159 for liquid and 157 for vapor from condenser 144 which unite in conduit 58 entering the upper portion of the oxygen column 59.

The liquefied argon at about -304 F. is removed by pipe 153 connected to passage 151. The rich fluid passing from liquefier 147 to the condenser 144 of the argon column by means of the pipes 154 and 155 is part vapor and at a temperature of about -308 F. is removed from the condenser in vapor and liquid phases by conduits 157 and 159 and then passes through conduit 58 into the oxygen column 59 about a third of the way down from the top. The oxygen which collects in the bottom of the argon column 140 is returned to the oxygen column 59 by conduit 161 which extends between the base of the argon column and a point in the oxygen column slightly below where the argon concentrate is removed.

The conduit 171 can be used to bleed off some gaseous oxygen from just above the pool at the bottom of column 59, if and when this is desired. The proper bleedoff can be controlled by automatic valve 173 in conduit 171. This valve 173 is preferably operated automatically in response to a determination of the composition of the gas at a suitable point in the rectifier such as indicated. The determination of gas composition is made by taking a gas sample from the withdrawal location in the oxygen column 59 by means of valved conduit 175 and passing the sample to a gas analyzer 177 connected to conduit 175. The automatic valve 173 is controlled by the gas analyzer 177 by suitable means 179 shown by a dash line on the drawing.

Reverting to the nitrogen production, the condenser 89 serves to separate out the neon and other similar gases since these gases will tend not to be condensed as does the nitrogen effluent. Since these gases are more volatile than nitrogen, they pass to the top of the condenser 89 and are removed by way of conduit 98. This separation of neon and other gases, besides being a Worthwhile recovery, also serves to maintain a high purity nitrogen product since the neon, for example, is removed from the nitrogen when it is purged from the system.

As mentioned at the start, the incoming air is compressed to about p.s.i.a. in compressor 13. At this pressure the air, of course, does not have sufiicient energy to provide the refrigeration required for the process; therefore, the well-known nitrogen refrigeration recycle is added to the system in a particular manner to furnish the necessary refrigeration. This nitrogen refrigeration cycle has been mentioned above in reference to the scrubber boiler 41 and condenser 89. A minor flow of the gaseous recycle nitrogen at about 270 F. enters this boiler 41 and, after boiling enriched liquid scrubbing air and so being liquefied passes through expansion valve 101 in con-duit 102 which is connected to boiler 41"and leads to nitrogen condenser 89. After expanding through valve 101 to about 4 atmospheres gauge pressure and moving into the upper section 91 of the condenser 89, this recycle nitrogen flow is completely evaporated by condensing the nitrogen efliuent entering condenser by means of conduit 87.

Afiter effecting the condensation of the nitrogen effluent in condenser 89, the evaporated recycle nitrogen or minor flo w of nitrogen leaves condenser 89 through conduit 105 and is divided into two parts by means of conduit 107 which branches off from conduit 105. The part of the evaporated recycle nitrogen which flows in conduit 107 having control valve 108 is utilized in the nitrogen recycle in a manner which will be subsequently explained. The residual part of the evaporated nitrogen which flows on in conduit 105 passes to the non-reversing passage 27 of the reversing exchanger 23 to effect part of the previously mentioned cooling of the incoming air, the

remainder of which cooling is done by the Waste nitrogen flowing in either passage 25 or 21. A small portion of the recycle nitrogen flow in passage 27 is withdrawn through conduit 109 at a point approximately two-thirds of the way up the exchanger from the entrance of conduit 105. This Withdrawn nitrogen passes through conduit 109 and its control valve 110 and enters passage .131 of the exchanger 123 at a point opposite where the compressed nitrogen to be expanded leaves passage 121 for expander engine 127. The reason for this withdrawal will be explained shortly. The major or larger portion of the residual part of evaporated recycle nitrogen continues through the passage 27 of the exchanger and leaves through conduit 111 having a control valve 113. This larger portion of recycle nitrogen next joins an augmented major flow of recycle nitrogen moving in conduit 115 which leads to the nitrogen recycle compressor 117 and thus forms a full nitrogen recycle flow. The full flow is compressed by, and discharged from compressor 117 into conduit 119 at pressure of about 2500 p.s.i.a. Conventional water-cooled heat exchangers (not shown) lower the temperature of the recycle nitrogen to about 80 F. Corn duit 119 passes the nitrogen to passage 121 of the high pressure recycle exchanger 123 where the full flow is initia1ly cooled by the augmented, expanded major flow.

At a point in passage 121 where the compressed nitrogen is at a temperature of about -3-0 F., a major flow of the high pressure stream or full flow of nitrogen is Withdrawn through conduit 125 and expanded more or less isentropically in a conventional expansion engine 127 to about 4 atmospheres gauge with the performance of external work. This cold expanded major flow is directed by conduit 129 to passage 131 of the high pressure recycle exchanger 123 where it eifects cooling of the counterflowing compressed minor flow of nitrogen in passage 121. In this manner the minor flow of the compressed nitrogen which remains after the major flow leaves passage 121 for expansion, continues through passage 121 and is further cooled. From passage 121, this minor flow passes through conduit 133 having valve 134- which suitably reduces the pressure of the nitrogen (preferably to about 160 p.s.i.aI), to the boiler 41 in scrubber 39 where it is further cooled as above described. As above mentioned, the throttled minor flow is divided after condenser 89 into :two parts at the juncture of conduits 105 and 107. The part in conduit 107 is added to the exhaust from the expansion engine 127 and thereby forms the augmented expanded major flow which moves through passage 131 while the residual part flows on in conduit 105 to reversing exchanger 23 and functions in the manner above explained.

The withdrawal of the small portion of the residual part of the evaporated minor fiow of nitrogen from passage 27 of exchanger 23 by means of valved conduit 109 is done in order to provide, indirectly, the temperature conditions in reversing exchanger 23 which will permit.

proper deposition and effective removal of high boiling impurities, such as carbon dioxide. It is to he noted that this withdrawn nitrogen passes through conduit 109 and its control valve 110 and enters the augmented expanded nitrogen passage 131 of the high pressure nitrogen exchanger 123 at a particular point. This point of introduction corresponds to the location in the exchanger 123 at which the major flow of nitrogen is divided from the compressed full flow for delivery to the nitrogen expander 127.

An interconnecting conduit 212 and control valve 214 are provided to connect conduit 83 to conduit 102 so that, when desired, nitrogen from the top of column 37 maybe added to the recycle nitrogen in conduit 102 to overcome any small leakage loss in the nitrogen recycle.

In a plant which has been built and successfully oper ated in accordance with the invention, there has been obtained continuous productions of liquid nitrogen of purity 99.999 percent or higher, liquid oxygen of purity 99.75 percent or higher and raw liquid argon of purity 98%. Using the method of the invention, such a plant is capable of recovery of 80 percent or more of the argon in the processed air; together with commercial yields of neon-rich gas and liquid output rich in krypton and xenon.

While illustrative forms of apparatus, methods and processes in accordance with the invention have been described and shown herein, it will he understood that numerous changes may be made without departing from the general principles and scope of the invention.

What is claimed is:

1. The process of simultaneous production oi high purity nitrogen and argon from air with high percentage recovery of the argon content of the processed air, comprising the steps of delivering a process air stream to a nitrogen rectification column and rectifying said air stream to produce an oxygen-rich liquid fraction and a high purity nitrogen gas fraction, delivering said oxygen-rich liquid fraction to an oxygen rectification column and separating therein an argon concentrate and a liquid oxygen fraction, drawing oft, heat exchanging, and compressing a portion of waste nitrogen from the oxygen column, cooling the compressed Waste nitrogen by heat exchange as aforesaid between uncompressed and compressed portions of the waste nitrogen, liquefying the compressed and cooled waste nitrogen by heat exchange with liquid oxygen from the oxygen column, condensing the high purity nitrogen eifluent from the nitrogen column, supplying a portion of the condensed high purity nitrogen eiiluent to the nitrogen column as reflux, drawing oil as a product a material portion of the condensed high purity nitrogen efiiuent, and supplying to the top of the oxygen column as reflux in the rectification of the enriched liquid air said liquefied waste nitrogen together with the remainder of the condensed high purity nitro-' gen efiluent, whereby the addition of the liquefied waste nitrogen to the reflux substantially compensates for the product nitrogen drawn off, thereby maintaining the optimum quantity and purity of the reflux as required for maximum argon recovery.

2. The process according to claim 1, together the additional step of further supplementing said reflux to the oxygen column by adding thereto a moderately pure liquid nitrogen fraction condensed in said nitrogen column.

3. The process according to claim 2, together with the additional step of subcooling said supplemented reflux by heat exchange with said waste nitnogen gas from said oxygen column and by heat exchange with said product nitrogen.

References Cited in the file of this patent UNITED STATES PATENTS 2,180,435 Schlitt Nov. 21, 1939 2,433,508 Dennis -2 Dec. 30, 1947 2,482,304 Van Nuys Sept. 20, 1949 2,530,602 Dennis Nov. 21, 1950 2,547,177 Simpson Apr. 3, 1951 2,552,451 Patterson May 8, 1951' 2,729,954 Etienne Jan. 10, 1956 2,762,208 Dennis i Sept. 11, 1956 2,824,428 Yendal l Feb. 25, 1958 2,833,127 Vesque et a1. May 6, 1958 2,960,838 Denton Nov. 22, 1960 2,982,107 Smith May 2, 196 1 

1. THE PROCESS OF SIMULTANEOUS PRODUCTION OF HIGH PURITY NITROGEN AND ARGON FROM AIR WITH HIGH PERCENTAGE RECOVERY OF THE ARGON CONTENT OF THE PROCESSED AIR, COMPRISING THE STEPS OF DELIVERING A PROCESS AIR STREAM TO A NITROGEN RECTIFICATION COLUMN AND RECTIFYING SAID AIR STREAM TO PRODUCE AN OXYGEN-RICH LIQUID FRACTION AND A HIGH PURITY NITROGEN GAS FRACTION, DELIVERING SAID OXYGEN-RICH LIQUID FRACTION TO AN OXYGEN RECTIFICATION COLUMN AND SEPARATING THEREIN AN ARGON CONCENTRATE AND A LIQUID OXYGEN FRACTION, DRAWING OFF, HEAT EXCHANGING, AND COMPRESSING A PORTION OF WASTE NITROGEN FROM THE OXYGEN COLUMN, COOLING THE COMPRESSED WASTE NITROGEN BY HEAT EXCHANGE AS AFORESAID BETWEEN UNCOMPRESSED AND COMPRESSED PORTIONS OF THE WASTE NITROGEN, LIQUIEFYING THE COMPRESSED AND COOLED WASTE NITROGEN BY HEAT EXCHANGE WITH LIQUID OXYGEN FROM THE OXYGEN COLUMN, CONDENSING THE HIGH PURITY NITROGEN EFFLUENT FROM THE NITROGEN COLU UMN, SUPPLYING A PORTION OF THE CONDENSED HIGH PURITY NITROGEN EFFLUENT TO THE NITROGEN COLUMN AS REFLUX, DRAWING OFF AS A PRODUCT A MATERIAL PORTION OF THE CONDENSED 